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My ISP's DNS servers are sometimes sketchy, so I use and respectively. The Layer 3 DNS server and Google's DNS server.

Sometimes there will be a long delay in resolving, and if I revert to the DNS servers given with DHCP, then everything will be fine for a bit, before I ultimately have to revert back to the 3rd party DNS servers.

Can someone explain this behaviour, or the reasons why this would happen?

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Too vague to really provide an answer. Sometimes their servers are busy and slow? – icky3000 Mar 25 '11 at 0:34
layer 3 and googles servers are too busy and slow? unlikely. I'm wondering if it's something to do with the isp still filtering out dns queries, which only works with their own dns servers.... – Jay White Mar 25 '11 at 0:42
Why is that unlikely? The well-known public dns servers are very heavily trafficked. – MDMarra Mar 25 '11 at 1:00
I read an article recently (where I can't recall) that implied that due to the number of DNS clients that use Google's public DNS servers that they may in fact slow down your DNS queries if you choose to use them. – joeqwerty Mar 25 '11 at 1:01
OK, fair enough, what I don't understand is the correlation. It's repeating, which makes me think it is not meaningless. My ISP's DNS servers are slow the majority of the time, and only seem to function when the public servers slow down for me. – Jay White Mar 25 '11 at 2:07

Sounds like UDP packet loss on the request, which causes a delay before a request is sent to the other server. The Level3 server might be flaking out - the Google server won't ever get hit unless that top server is failing to respond to queries.

Put some junk IP that won't respond in for your primary name server, and put a valid one in the secondary - then run an nslookup. If that's the kind of delay you're feeling, then it's probably an issue with your primary name server.

That or an issue with UDP packets making it out in general - is your connection generally congested?

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